WIP: Add window opacity slider tool

[personalizedrefrigerator]

Summary

see also #3312.

• Implements suggestion 10 from 19 Ideas including suckless to terminal stuffs #3259
  

To-do

• Error handling for when the user doesn't have a compositor running.
• Remove code duplication (maybe ZoomSlider and the opacity slider should share a slider base-class?)
  • AbstractSliderItem (currently used by just the opacity slider)
• Setting for whether to make the toolbar transparent, in addition to the Xournal widget.
  • Potential reference: https://stackoverflow.com/questions/16832581/how-do-i-make-a-gtkwindow-background-transparent-on-linux

[personalizedrefrigerator]

For now this is just a duplicate of the zoom slider's icon (but I think it works for this).

[personalizedrefrigerator]

In ToolMenuHandler::initToolItems, control->getWindow() returns nullptr. As such, we can't pass the MainWindow* to the ToolWindowOpacitySlider directly. I don't like giving it the entire control object, though...

[rolandlo]

This looks extremely fun, but wouldn't it make more sense to just make the XournalWidget transparent?

[personalizedrefrigerator]

make the XournalWidget transparent?

From what I've found online, it looks easier to make the entire window transparent (So I'll have to look into how to do that...).

For my use case, though, making the entire window transparent is (slightly) more useful, as I plan to use this feature to take notes on video recordings of lectures. Making the entire window transparent lets me see content that would otherwise be hidden by toolbars. Of course, using the empty toolbar is another option!

Additionally, the slider does not allow the user to make the window entirely transparent and the window becomes fully opaque again on startup. As such, this shouldn't force the user to search for an invisible slider to make the window visible again.

[kneardhead]

Man I didn't think this will happen so quick! Thanks I truely appreciate!

The opacity thing looks very cool not gonna lie.

[rolandlo]

make the XournalWidget transparent?

From what I've found online, it looks easier to make the entire window transparent (So I'll have to look into how to do that...).

For my use case, though, making the entire window transparent is (slightly) more useful, as I plan to use this feature to take notes on video recordings of lectures. Making the entire window transparent lets me see content that would otherwise be hidden by toolbars. Of course, using the empty toolbar is another option!

I have a different use case in mind: displaying some image or other information in a window below Xournal++ and using it to copy the image/info to Xournal++ by drawing the outlines etc. that shine trough. In German there is the word "abpausen" for it, see this video if my explanation was not clear. While doing that, I would like to have the toolbar normally visible, because otherwise it's a bit hard to find the right tools/colors etc.

Maybe it could be made a setting, whether the whole window is made transparent or only the XournalWidget.

Additionally, the slider does not allow the user to make the window entirely transparent and the window becomes fully opaque again on startup. As such, this shouldn't force the user to search for an invisible slider to make the window visible again.

[LittleHuba]

maybe ZoomSlider and the opacity slider should share a slider base-class?

If there is duplicated code, most definitely!

[personalizedrefrigerator]

This was the case before this PR, too.

[personalizedrefrigerator]

I'm going to wait for #3312 to be merged/finished before finishing up this PR.