UI Improvements

[letehaha]

There are several places in the app's UI that should be improved:

1. /transactions page – we need one more filter to filter by categories. It should work the same as accounts selection – multiple options available, or "all". Yet the design should differ and probably replicate the one from the "Select category" field from the tx creation form with the additional ability to multiselect categories. To represent it on the UI side, I'd make a suggestion to add a checkbox next to the cateogory name on the right side.
   Yet, the design is discussable. There's also an idea to show category selection is a separate dialog and list all caregories into nested expandable list, also with checkboxes.
   This also requires updating the backend to support filtering by categories.

2. Current accounts selector on the /transactions page looks terrible. We need some multiselect field, like select field with checkboxes and the search bar. Once accounts are selected, they should be displayed inside the field or below it. Again exact good-looking design is TBD.