Change: Add tooltips for text file buttons, and shorten Website / Readme buttons.

[PeterN]

Motivation / Problem

• The Readme and Changelog and Licence buttons that open the text file viewer don't have a tooltip.
• It is inconsistent that Readme is prefixed with "View", but Changelog and Licence are not.

Description

• Add tooltips for the these buttons.
• Change 'View Readme' to 'Readme', and also 'Visit Website' to 'Website' for consistency. As these buttons have tooltips there should not be any ambiguity with their meaning.

This is separate from the existing PR that rearranges the buttons themselves.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker