Add a submenu with multiple copy options to post menu

[Astridchan]

The "Copy text" option in post menu is replaced with a Copy submenu with the following options:

• Copy link (eg. >>6235023)
• Copy cross-board link (eg. >>>/g/6235023)
• Copy post text
• Copy post URL
• Copy image URL
• Copy links in post (opens the "Show links" dialog)
• Copy post info (opens the "Post info" dialog)

For the last option to make sense, the post info dialog was changed to have selectable text.

In action:

Other, smaller changes:

1. Post info now displays the subject and name, allowing the user to copy them to the clipboard.
2. Swapped position of filter and copy option so copy appears under the main menu and filter under "more..."
3. "Show links" option is only shown if there is at least one linkable in the post.
4. Long-press on a link under "Show links" would trigger a normal click and a long click. The fix was to make long click listener return true instead of false.
5. AlertDialog style was being applied to the context menu causing unwanted effects (see below), Fixed by changing android:background to android:windowBackground.