clipproperties: Creating a license bumper.

The license bumper creations feature should be in the same section of the UI as the
other clip creations features

The features settings were found in the settings of the title and color clips, which is
not where they should be.

To solve this issue, create a "Create Licenes Clip" button below the "Create Color Clip"
button that generates and selects a default license bumper on click.

Progress for Issue #2415
1 job for pitivi-license_bumper_generator in 33 seconds (queued for 1 second)
Status Name Job ID Coverage
failed build #1226547


Name Stage Failure
build Build
/build/pitivi/po/pl.po:727: keyword "HEAD" unknown
msgfmt: too many errors, aborting
ninja: build stopped: subcommand failed.
Uploading artifacts for failed job
Uploading artifacts...
WARNING: /builds/schumacherryan2001/pitivi/xunit.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1