make annotation_size_factor configurable from UI

[RobertMueller2]

Related to #106

• adds UI element to adjust annotation_size_factor config setting, using GTK SpinButton control. This affects the sizes of all tools and allows for finer control of the annotation sizes ;)
  • the SpinButton control offers by GTK default
    • left click to increase/decrease
    • right click to go to min/max
    • middle click to increase/decrease by a bigger jump
• adds reset button allowing to going back to the default/configured value

The original goal of the issue was to make the font/font size configurable. I think somebody can implement that later, for now this seemed like a much quicker win.

Still WIP:

• make reset button more visible, not sure yet what the issue is, according to GTK_DEBUG=interactive it has the same color as the others. Edit: it was "edit-reset" vs "edit-reset-symbolic"
• consider GTK drawer for the additional UI elements
• allow focus of the input box. i disabled this for now as it stole focus from the text annotation tool

Also, I'm not really familiar with Relm. I couldn't say if all of this is the best possible way to implement it. It does work here and didn't seem to break anything. ;) I have tried adjusting the APP_CONFIG with write but that led to deadlocks (or WouldBlock with try_write) -- since nothing else writes except for intially command line parameters and config, this might not have been the way to go anyway.

[RobertMueller2]

Revealing/hiding the elements now looks like this:

[RobertMueller2]

dealing with the focus in a consistent way, considering tab, enter, potentially escape, cursor up/down and pgup/pgdown is trickier than I thought. Even using the hide button as a possible focus target doesn't make it much better.

Either the text box of the spin button remains unfocusable, or this needs to go in a dialog.