Assign icons to iconless tool actions

This commit grants icons to tool actions that currently lack icons. This will mainly save UI space when a user configures their toolbars to include a particular tool action.

Icons have been assigned to the following actions:

• Assistant Tool
• Bezier Curve Tool
• Bezier Curve Selection Tool
• Contiguous Selection Tool
• Elliptical Selection Tool
• Freehand Path Tool
• Freehand Selection Tool
• Magnetic Curve Selection Tool
• Polygon Tool
• Polygonal Selection Tool
• Polyline Tool
• Rectangular Selection Tool
• Similar Color Selection Tool
• Text Tool
• Transform Tool

This commit also changes the icon assigned to the Edit Shapes action, as it was using the Bezier Curve icon.

Before:

After:

Test Plan

Confirm that each action is assigned to the right icon, whether it's through the Configure Toolbars interface, the Keyboard Shortcuts interface, or the command bar (Ctrl + Enter).

Confirm that these icons conform to Light/Dark themes (seems to require restart when an opposing theme is selected).

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.