GitLab

* Implement checkstyle rules for App Inventor

This commit implements checkstyle for App Inventor's Java code. The
App Inventor style rules are based on the Google style rules, with an
exception added for methods in the components/runtime package that are
annotated with `@Simple*` or `@Design*` annotations. The App Inventor
specific rules are placed at the end of the appinventor_checks.xml
file.

In addition, there is a script called checkstyle_git.py that is
written in Python 3 and will check the git diff against the rules by
running checkstyle on the changed files and crossreferencing the line
numbers with the checkstyle output. This can be used as a git
pre-commit hook to check that changes conform to the style guide. The
script returns -1 on error, which will abort the commit until the
changes confirm. This behavior can be bypassed by exporting the
BYPASS_CHECKSTYLE environment variable.

These two changes are made available as ant targets:

- `ant checkstyle`: Run the checkstyle_git.py script to check the
  git diff for problems.
- `ant checkstyle-all`: Run checkstyle on the whole repo (slow).

Change-Id: I26c6049bd49d1ce3f9b8dbd48f8f115a89fb5b52

* Revamp checkstyle-git script logic

Change-Id: Ie417057b2d009ba70279e70df102851b2f4552cd

* Add custom checks for EventDispatcher in @SimpleEvent

Change-Id: I72a704e9f8d489aa08cefd50ca180b1130b3e5fa