Add trashbin GUI

[raimund-schluessler]

Closes #29.

Todo:

• Load tasks of restored calendar
• Show deleted tasks and events
• When restoring a task, restore all children and all direct ancestors When restoring a task, restore all children and all direct ancestors #1691

[codecov]

Codecov Report

Merging #1675 (ec9d76c) into master (31c7e1b) will increase coverage by 34.76%.
The diff coverage is n/a.

❗ Current head ec9d76c differs from pull request most recent head 3f3a7f1. Consider uploading reports for the commit 3f3a7f1 to get more accurate results

@@              Coverage Diff              @@
##             master    #1675       +/-   ##
=============================================
+ Coverage     31.89%   66.66%   +34.76%     
- Complexity       35       41        +6     
=============================================
  Files            58       10       -48     
  Lines          2856      153     -2703     
  Branches        540        0      -540     
=============================================
- Hits            911      102      -809     
+ Misses         1945       51     -1894