Introduce SOURCE_PATH generator variable.

This means that the source path is no longer hardcoded as ${ROOT}/source.
Merge branch '6714-generator-refactor'
Merge branch '6714-generator-refactor'
Merge branch 'master' of https://github.com/qooxdoo/qooxdoo
Merge remote-tracking branch 'upstream/master'

Conflicts:
	tool/bin/generator.py
Merge branch '6714-generator-refactor'
Merge branch 'master' of https://github.com/qooxdoo/qooxdoo

Conflicts:
	tool/bin/generator.py
Merge branch '6714-generator-refactor'
Merge remote-tracking branch 'origin/master' into 6779-dry-base-json
Merge branch '6779-dry-base-json'
Merge branch '6714-generator-refactor'
Merge remote-tracking branch 'upstream/master' into 6779-dry-base-json

Conflicts:
	tool/data/config/base.json
Merge branch '6714-generator-refactor'

Conflicts:
	tool/data/config/base.json
Merge branch '6779-dry-base-json'
Merge branch 'master' of https://github.com/qooxdoo/qooxdoo
[BUG #7145] Added qx.emulatemouse environment key.
[BUG #7145] Make the focus handler listen to touch as well in case of qx.emulatemouse.
[BUG #7145] Altered the mouse handler and added the emulate mouse event handler.
[BUG #7145] Added unit test for the new env key.
[BUG #7145] Also fire mouse wheel events.