seperate Cheatsheets from help bundle

[fgdrf]

Right now the help bundle has required-bundle dependency to org.eclipse.ui.cheatsheets.
For uDig itself its uncritical but for RCP-Apps build on top or with uDig containing help, it should be an option to have cheatsheets.

In addition, there is an unused dependency to org.eclipse.ui.intro from help-bundle, please remove it too

This allows an integrator to choose if cheatsheets should be a part of a product or not.

affected bundles:

• org.locationtech.udig.help
• org.locationtech.udig.issues
• org.locationtech.udig.project.ui

expectation:
new bundles with cheatsheet content and extensions (e.g. org.locationtech.udig.project.ui.cheats for project related cheatsheets)