PAINTROID-440, 441, 442 Created the landing page and implemented the basic functionality

[CodeChamp-SS]

https://jira.catrob.at/browse/PAINTROID-440
https://jira.catrob.at/browse/PAINTROID-441
https://jira.catrob.at/browse/PAINTROID-442

• 1/3 of the top of the screen shows the thumbnail of the most recent project with the "edit" icon.
• When the project is opened, the back arrow leads the user back to the landing page.
• Landing page has 2 floating action buttons:
  -> The main button has the plus icon. When pressing this button a new empty project will be opened (same as "new image" in the overflow menu of the current state of Paintroid).
  -> The second button above (same diameter) has the download icon. When pressing this button the same thing as with load an image from the overflow menu of the current state of Paintroid happens (An Intent opens to select an image from the phone's storage).
• All the projects of the user are shown in a list view below the most recent project, sorted according to the lastModified date in descending order.
• There is an overflow menu with options to delete, and show details(image metadata) for each project.
• Used Floor database for storing the metadata of projects.

[juliajulie95]

Please check the failing flutter analysis

[ThorstenBandel]

LGTM thanks!