AbstractAppContainer: Use ProgressButton

[cassidyjames]

This adds a progress bar to the button instead of packing a whole Gtk.ProgressBar. As a result, the UI doesn't resize. Fixes #1653, fixes #1698.

I initially investigated using a Dazzle.ProgressButton, but in testing, it wasn't styled correctly (and it seemed overkill to add Dazzle as a dependency for something relatively simple). Instead, this branch makes a new simple ProgressButton using a similar approach to that of Dazzle.ProgressButton.

• Are we okay putting the technical details in a tooltip?
• Check/add RTL support (Dazzle looks to have this in its custom styling)
• Decide if we should use the system @accent_color or something else

[cassidyjames]

This appears to work as well as the progress_bar in master with the fixes noted above. There are probably ways it could be improved, though.

[danirabbit]

From a design perspective, this looks good to me. I'm +1 on having the details in a tooltip

Probably does need a little :dir(rtl) style selector thingy

[cassidyjames]

@danrabbit alright, RTL is handled and I pulled the non-changing CSS out into a GResource so we're only changing the one line. Looks and behaves the same as before (with the addition of RTL support).

[danirabbit]

Tested this in the various views and it looks good and works as expected. Fixes the issues with the progressbar in the layout. Nice work!