Try to create test suite

SOMEONE KILL ME PLEASE.
add todo.md
missing-list-entry: Fix indentation
remove widgets binary
CMake: Try to correctly determine the gresource deps
Cmake: Ditch exact resource dependencies

Seriously, it seems to be completely impossible to do this right. I just
wasted a few hours on this and I'm really not eager to waste any more
time.
tests/missing-list-entry: Add buttons to change state
Rename the widgets test to missing-list-entry
tests/missing-list-entry: Remove old comments in cmake file
Corebird.vala: Also print debugging messages to stdout

...if __DEV is set, i.e. -DDEBUG=ON was passed to cmake
Add imagedialog test case

And move the redundant stuff into tests/CMakeLists.txt
ImageDialog: Correctly close the save dialog on cancel
MissingListEntry: Don't take a id parameter in the constructor

DefaultTimeline will always keep a MissingListEntry around so we set
the id(now lower_id) when we find the stream interrupted.
Merge branch 'continuous' of https://github.com/baedert/corebird into continuous