File browser no longer throws ArgumentException when entered filename…

… contains invalid characters; filename input field turns red instead

Fixed performance issues that occur when selecting lots of files

- Added context menu that can be opened in 2 ways: a) right clicking …

…a file on standalone platforms or b) clicking the More Options button located at the top-right corner. Context menu has the following buttons:

  - Select All (only when multi selection is enabled and More Options button is clicked)
  - Deselect All (only when more than 1 items are selected and More Options button is clicked)
  - New Folder (closed yasirkula#25)
  - Delete (only when 1 or more items are selected)
  - Rename (only when 1 item is selected)

Bumped version

Added multiple file/folder selection support (closed yasirkula#10)

Note that the Scripting API has also changed slightly with this new feature. The OnSuccess callback now returns string[] instead of string and dialog functions now take an allowMultiSelection parameter (false by default)

Added OpenUPM instructions

Fixed yasirkula#18

Fixed yasirkula#18

- Fixed yasirkula#16

- Fixed a possible NullPointerException on Android when selecting a new folder using SAF

Updated README