Replace GoPackageListingPureWidget with use of more general SearchableListWidget.

Use GetGoPackages2() that returns *GoPackage rather than
ImportPathFound.
Fix up GoPackageListingPureWidget before removing it.

Add Godeps file to be able to run this version more easily in the
future.
Remove deprecated GoPackageListingPureWidget.

Remove Godeps too because I'm not planning to keep it updated (instead,
I will be keeping the code up to date and compatible with tip). It's
only for occasional checkpoints atm.
Replace ImportPathFoundSelector with GoPackageSelecter.

Commented out doc.Package-based node, because it duplicates AST stuff
from other packages, and doesn't point back to real code... Instead, I
mimic doc.Package functionality, but stack it on top of types.Package
rather than a duplicated ast.Package.
Get GoPackage FullPath more directly.

There's already a field for it in build.Package.
Update to match API change.
Fix binary search off by one in caretPositionInternal.TrySet().
Work on adding a popup menu to TextBoxWidget (for goto symbol).
Add a quick and naive implementation of semantic highlighting.

Very basic implementation that can be much improved (e.g. group
identifiers into coherent categories, distribute colors equidistantly
among an equal-brightness spectrum, etc.).
Thanks to https://medium.com/p/3a6db2743a1e/ for inspiration.
Tweak colors.
Work on getting popup to close on escape.
MVP of a "go to symbol" popup menu mostly working.

Unified variable naming differences between two "Render only visible
lines" sections.
Enable FilterableSelecterWidget triggering SelectionChanged when typing.
Add TypeUnderCaretWidget, labels for CollapsibleWidget.

Turn off semanticHighlighting for now, until a more sophisticated color
picking algorithm is done.
Fix NewGoonWidget to panic if it's not given an address of variable.
Tweak comment highlighting to be darker green.
exp11 fixed and /inline/ functionality restored.
Add leading tabs on enter, tweak colors, use DrawInnerBox for buttons.
Remove `Bool` type; add comments.
Rename Strings2 -> SliceStringer.
Fix potential debug panic because of MultilineContentFuncInstant unsafeness.

Thanks to @github for hosting a mean party! <3
Test live vcs status of a single repo.