Design Rework #1 - App icon & texts

[LuisMossburger]

Description See also #100 and #118 for progress. This merges reworked texts with a shared tone that matches our "company values" and the new app launcher icon. Please see the single commits for more information, as more things were done here. In short:

• Proper exceptions for singular/plural actions (e.g. "Regal löschen?" and "Regale löschen?" instead of "Regal(e) löschen?")
• Use a consistent friendly and short tone for all texts that matches our company identity we created on the beginning
• Deleted unused texts, restructured strings.xml
• Updated bookForm to use standard "*" for mandatory fields and moved ISBN field down (we talked about this in team meeting, basically: if I need to use manual adding, not having an ISBN is probably the reason, so this field should not be on top)
• Bugfix for deleting authors (multiple at once not possible) and notes (were not updated after deleting)

Next up: Reworking the help, then colors & UI elements.

Affects Texts in multiple fragments.

Notes for Reviewer Please review & comment/approve - maybe don't be too nitpicky about formulations as I think the changes will need to be discussed in a future team meeting instead of between the two of us here :)

[LuisMossburger]

Thanks for the review! Adjusted everything and additionally also changed the ID of the "mandatory" text in the book form.