-Since multi-selection of characters is now possible, it's more intuitive that all selected characters should be deleted.
-Added a confirmation dialog before character-deletion? Why the heck was this not a thing before? It was way too easy to accidentally delete characters, with no way to restore them without external source-control or backups.
Also, a change has been made to the character tree view. Theoretically it shouldn't have any functional significance, but "addCharacterInfo" is used for adding info to only one character, so it makes sense to remove the old multi-selection info-adding functionality, even if it wouldn't ever be accessible.
-Added currentCharacters method, a list counterpart of currentCharacter.
-Replaced the connection for currentItemChanged with itemSelectionChanged. This is to accommodate for the bulk-changes when multiple characters are selected. The currentItemChanged method is now not called with a connection, but from the handleCharacterSelectionChanged method. The latter is called with the itemSelectionChanged connection.
-If no valid characters are selected, the tabPersos widget is disabled. This obviously breaks the functionality of bulk-adding info to characters. This will have to be fixed by only disabling the parts of the tabPersos widget that should not be affected with multiple selection operations.
-Changed the selection mode to extended selection on the lstCharacters tree-view. This will not affect anything else other than the "detailed info" rows. Every other change to a character's descriptions, motivations and such only affect the last selected one.
-Made a method to get all the IDs of the selected characters.
-Added a character-info dialog UI. Originally, adding information worked by adding a placeholder and then changing it. Users never want to just add a placeholder without initialising the values. And the bulk-adding only works this way.
When creating a new character, sets an appropriate importance level.
* If a character is selected, the new character has the same importance level.
* If a top-level importance level is selected, the new character has that level
* Otherwise, the importance level is zero
