Rename PiTiVi to Pitivi

This improves our branding, which is a surefire way to world domination.

Also update the source code file headers to drop the "non-linear" term,
as it is an anachronism. With software nowadays, any video editor is non-linear,
so that term is redundant.

Dropping the "camel case" fixes bug #705756
utils/pipeline: Make the viewer's back/forward seek buttons work again
pipeline: Wait for async done before launching next seek
Fix broken tags in Slovenian help translation
ruler: Seeks need to be on idle to compress the seeks while scrubbing
utils/pipeline: Save the position of the last compressed seek

This ensures we actually seek to the correct position, the one requested last.
Put multiple new clips in sequence on the same layer

Earlier the code would put all clips on top of each other and give each
clip its own layer. This is unexpected and not very helpful.
Updated Hungarian translation
Updated Lithuanian translation
Updated Slovenian translation
utils/pipeline: Emit a loggable warning when attempting recovery
project, timeline: Set restriction caps on tracks.
project: update tracks restriction caps when framerate is changed.
data/icons/Makefile.am: correctly name the uninstall-hook
tests: remove test_cache as it is outdated.