Github: rewrite CreatePullRequestAction

* split doalog window onto two:
 - select target repository
 - select target branch | show diff | create PR
* fetch remote only once
* do not allow custom target refs (ex: tags, hash)

Changing target repository seems to be very rare operation, and it's nice to reduce complexity of main window.
Github: rewrite CreatePullRequest: Part2

* do not recreate window on changing target fork
IDEA-114360 Github: remove double borders for TextArea inside ScrollPane
Github: remove inspection warning
Github: remove useless 'final'
Github: get GithubAuthData on worker init
Github: rename variable
Github: disable 'ShowDiff' button on fetch fault
Github: move ProjectSettings usage
Github: rename variable
Github: change default value only after successful loading
Github: use dialog manager
Github: use getRequestTitle() instead of getTitle() - typo
Github: fix tests
Github: use prefix for @TestOnly functions
Merge branch 'master' into createPR

Conflicts:
	plugins/github/src/org/jetbrains/plugins/github/GithubCreatePullRequestAction.java
Github: do not hang on cancel initial showTargetDialog
Github: change labels
Github: change error message
Github: remove static import