rose edit: Load configs on demand

Fix #622

This introduces load on demand functionality to `rose edit`, making the following changes:

 * a suite with more than 10 apps has the apps loaded in in preview mode
 * save has been modified to only save out non-preview apps
 * right-click menu fixed to display all options for nav-panel
 * right-click menu disabled for preview apps
 * splash screen is put up while the app is being loaded in N.B. followup in #731 to remove the need for the necessary hack used here
 * splash screen now calculates fraction correctly
 * CLI switches added to `rose edit`: `--load-on-demand` and `--load-all-apps` to override the new default previewing behaviour
 * preview apps are displayed in grey underline - similar to latent items