Add categories

[talvasconcelos]

Add categories to products and filtering by category

Closes #37

[prusnak]

I found two issues:

• one in categories itself - if categories were not set, the attempt to create a new TPOS failed - this is addressed in b4c87b3 (which also modifies the Categories editor to behave exactly like the Tip editor - it uses the same attrs passed to q-select now)
• filtering was wrong - if no categoryFilter was given, the searchTerm stopped working - this is addressed in 8d7ee8b which refactors the filteredItems function

And one nit:

• let's select ALL button when no categoryFilter is set - addressed in 0112a0e

Please test my changes whether I did not break something else.

Otherwise I am really happy with this PR!

[talvasconcelos]

Tested and all works as intended. Thank you for your input @prusnak

[motorina0]

utACK

[dni]

utACK