* refactor: add libtransmission-app
* refactor: add libtransmission-app/display-modes.h
* refactor: use app::SortMode, app:ShowMode in Qt client
* feat: add to_variant(), to_value() in serializer
* refactor: use app::SortMode in GTK client
* refactor: use app::ShowMode in GTK client
* refactor: make naming consistent with libtransmission-app
Move `FavIconCache` into a new `transmission::app` namespace.
I intend to add a couple more pieces into libtransmission to avoid
code duplication between the Qt and GTK apps.
We should consider making another module for these pieces if they start
to pile up; but for now, let's cordon them into their own namespace.
* chore: rename tr_strvContains to tr_strv_contains
* chore: rename tr_strvStartsWith to tr_strv_starts_with
* chore: rename tr_strvEndsWith to tr_strv_ends_with
* chore: rename tr_strvSep to tr_strv_sep
* chore: rename tr_strvStrip to tr_strv_strip
* chore: rename tr_strvToBuf to tr_strv_to_buf
* refactor: rename tr_saveFile() to tr_file_save()
rename tr_loadFile() to tr_file_read()
rename tr_moveFile() to tr_file_move()
* refactor: rename tr_parseNum() to tr_num_parse()
refactor: rename tr_parseNumRange() to tr_num_parse_range()
* chore: group related functions together in header
