Duplicate shortcut for 'Selection mode'/'Synchronize viewers'

[petebankhead]

Bug report

Describe the bug
There's no way accelerator for Selection mode... which isn't itself a bug, until you look at the code and see that there should be, it is just the same was the one used to synchronize viewers.

See https://forum.image.sc/t/feature-request-keyboard-shortcut-for-selection-mode/45154/3

Expected behavior
Accelerator for Selection mode works as intended.

Desktop (please complete the following information):

• QuPath v0.2

Additional context

A few things should probably be done:

• Set Shift + S as an accelerator (need to check this isn't used elsewhere...)
• Make Selection mode available in a menu (so its accelerator is displayed)
• Log a warning if an accelerator is defined multiple times (within QuPathGUI.registerAccelerator)
• Optional: Make it easier to define custom accelerators for actions through QuPathGUI (see workaround script posted on the forum)

[petebankhead]

Fix has been merged, will be in v0.3.