Implement placeholder text for input fields and get rid of hack with mod filter

different font colour
no need to delete the text, when one wants to write there something
text displayed is not a value so field is technically empty
will disappear when user writes some character
will appear when user deletes all characters
we can get rid of text value "Filter" in mod selection screen and ugly hack around it

Differential Revision: D2460
Comments by: vladislavbelov, elexis