Issue: XF2.x: Enhance categories/attachments selection in overlay: Change +/- elements to iconic checkboxes (plus two other related suggestions)

1) Add attachments to page & Add page to categories overlays currently use +/i icons for vw-search-add/vw-search-remove for the vw-search-the list. I think XF 2.x looking checkboxes would be more appropriate here (and consistent with XF). To be honest I am not a fan of using the +/- icons to represent select/deselect, particularly since the 'expand folder' image also includes a small +. Clicking the folder + performs a completely different action to the other other + icon (not a bug, but let's just remove the potential confusion ).

Please view the two attachments (one original, one edited) to better understand the change that I suggest. 'Selected row' colours used are just examples. The selected row checkbox color is the mutedTextColor property and the background is the inlineModHighlightColor property (which, despite its name, is exposed to general users in some places such as XFMG for managing own media content).

Some of this may also be relevant to books and their chapters, plus wiki content integration.

2) Additionally, when results are (optionally) grouped by prefix/area/category, then perhaps the current folder expanded/collapsed folder icon images could be replaced with FontAwesome chevron icons (up/down), just like XF2.x does with its categoryList-toggler element (see core_categorylist.less). A public example of the categeryList-toggler is the XFMG category list (Test media): https://xenforo.com/community/media/.

3) "Select All Results" checkbox could also be switched to the iconic version.