Implemented shortcuts for annotating while taking screenshot

To enhance the UX, a few handy shortcuts were suggested with this MR. For the process of taking screenshots, the following new features were implemented:

Select tools: every annotating tool can be activated with a shortcut of the tool's first letter, i.e. A for Arrow, or F for Freehand and so on.

Change tool size: scrolling the mouse wheel now changes the size (usually it is the stroke) of the tool. Important to note, that for Text and Number it is font size that changes, and for Blur and Pixelate — strength.

Change tool color: the Space shortcut opens the main color dialog for tools (which is the color of stroke for Freehand, Highlighter, Line, Arrow; of text for Text; and of fill for Number). Ctrl+Space opens the secondary color dialog (which is the color of fill for Rectangle, Ellipse; and text for Number).