PAINTROID-432: Implement clipping tool to erase background more easily

[Electronix1337]

This PR is the main work of my GSoC 2022.

I've Implemented a new tool called "clip area/clipping tool" with which the user is able to mark an area and when applying the tool or switching to another tool with a marked area already drawn, everything except the marked area will be erased. The path for the marked area can have any color but depending on the tool color, the path either has a white contour (when black is the current tool color) or black otherwise.

In order to have an actual closed area to properly clip I decided to do the same as i.e. Photoshop and just draw a last line from the last coordinate (where the user released the finger) to the initial coordinate to have an actual closed marked area to clip. I did not find out yet why sometimes the line is an actual line and sometimes the patheffect of the toolpaint actually applies and a dashed line will be drawn.

• When marked area is already drawn and user tries to draw another area the old area will just be deleted and the new area will be drawn on the canvas.
• When marked area is drawn and user presses undo, the marked area will just be deleted.
• When marked area is drawn and applied and user presses undo, the bitmap before clipping will be displayed, as the path command for the marked area will be deleted from the undo/redo list, also color changes will not be undo/redo-able as discussed with Wolfgang.

Click for demo on black bitmap with undo and redo:

Click for demo on pink bitmap with redrawing marked area and color change:

Click for demo on actual face:

https://jira.catrob.at/browse/PAINTROID-432

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[FromG]

pls change the requested changes.

[FromG]

lgtm

[wslany]

Thanks!