Rebase xubuntu-logo plymouth theme on ubuntu-logo

[bluesabre]

The ubuntu-logo and xubuntu-logo themes have diverged significantly. This (WIP) PR replaces the current xubuntu-logo theme with ubuntu-logo and replaces assets.

Visual differences between the themes:

• Progress dots instead of spinner
• Logo moved to bottom to enable display of OEM logo
• Larger message area for system messages to be displayed

Plymouth features enabled:

• Plymouth.SetBootProgressFunction
• Plymouth.SetDisplayQuestionFunction
• Plymouth.SetQuitFunction

Todo:

• [ ] Update theme colors (xubuntu-logo.script)
• [ ] Verify logo positioning / sizing
• [ ] Update password-field.png
• [ ] Update password-field16.png
• [ ] Update progress-dot-off.png
• [ ] Update progress-dot-off16.png
• [ ] Update progress-dot-on.png
• [ ] Update progress-dot-on16.png
• [ ] Update / drop xubuntu-logo.grub
• [x] Update xubuntu-logo.png
• [x] Update xubuntu-logo16.png
• [ ] Delete xubuntu-logo-old

Resolves #5