[w1tcherrr]

Updates ValvePak to 7.2 and adds VPK editing to the GUI.

• Create a VPK, create one from a folder, or edit an existing one, both single file and split into chunk files.
• Add files and folders (several folders at once), create, rename and remove them, and save in place or under a new name. Files that already exist can be replaced or skipped, and saving and adding run behind a progress window.
• Editor tabs with unsaved changes show * in the title, Ctrl+S saves, and closing asks to save.
• VPKs inside VPKs are read in place at any depth, in the GUI and the CLI.
• --vpk_create packs a folder into a VPK, optionally split into chunk files, with the same path and file name checks as the GUI.
• Searching file contents reads chunk files through archive streams instead of copying them into memory.
• The VPK guide covers creating and editing, with new screenshots.

Also fixes:

• New VPKs no longer get an empty README.txt.
• Find file contents in single file VPKs at the right offsets, including files with preload data.
• Recovered deleted files stay out of the package entries.
• --vpk_cache only records files that were written, so failed ones are retried on the next run.
• Shader collections are read through memory mapped streams instead of copies.