Improvements to GlobalShortcuts

[Spacerat]

This PR:

• Makes the GlobalShortcuts utility a bit more flexible and clean by leveraging CodeMirror's key-combo logic instead of my half-baked home-grown solution
• Moves GloalShortcuts into a separate file
• Changes the FuzzyPrompt to use GlobalShortcuts
• Gives names to the functions in cm.options.extraKeys

These are enabling changes for the shortcut preview dialog, but they're useful even without it.

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Since making this PR I have expanded it, it now contains some bugfixes and new features.

Save-shortcut fixes:

• If you start cloudpebble and open two files in order, file1.c and file2.c, and then switch back to file1.c and then switch to the compilation pane and use the save shortcut, file2.c would save.
• Resources behaved in a similar way except they would just throw an exception instead of saving.

I've made it so cmd-s will save the last source file you have/had open, or the current resource you had open, or do nothing

Small Error handling fixes
There are certainly many more changes like this needed, but this goes a small way to helping. In various places, errors are swallowed by error handlers when they are not rethrown in catches and not displayed anywhere, which makes user feedback and development tricker. I added two throw statements, and added the ability for the main cloudpebble loading bar to show an error state.

FuzzyPrompt command subsections

I added a subsection system to the FuzzyPrompt and added the capability to show keyboard shortcut hints, which should make it easier for users to discern what does what and discover keyboard functionality. I added a few file editing commands to this prompt to show off a few of the more obscure functions (e.g. auto-indent, save all, auto-comment); it wouldn't be too hard to add more.

When searching, the first section which is shown is the section corresponding to the first search result, the second section which is shown is the section corresponding to the first search result which is not in the first section, and so on. Within each section, items are ordered by search score first and then alphabetically.