#71 Record filtering, paging, sorting

[ledsoft]

Resolves #71.

Some additional notes:

• The filters are displayed in a popover, because they would not fit in the table cells. Different visualizations are up for discussion (e.g., a line of filters above the table).
• Export now respects the filters, so only records matching the filters are exported.
• I did some additional refactoring (mainly giving buttons a common min width) that was not part of this ticket. Sorry.

[blcham]

Depending on how hard it is to implement (let's discuss), I suggest the following improvements:

1. Change "Disabled filtering" icon as it looks strange:
   

2. Put filter also to headers similar way as it is done in google sheets:
   

3. It would be nice to know how many records there are in total

4. It is strange to scroll within the browser and then use paging of records. Thus, it would be nice if the page of records fits exactly on the page of the browser, so the user would not need to scroll to find the button for the next page. The alternative is to have: a) infinite scrolling without paging; b) a configurable page size.

[ledsoft]

@blcham Updated the PR according to your feedback.

[blcham]

LGTM, works and looks very nice