Refactor treeview classes

[marc-vdm]

Issue

Treeview classes for Methods and ISIC activities should be updated:

• Refactor to have proper superclass, lot of functionality is double between Method/ISIC classes
• Redo filter/search from Methods class to be in pandas DF like implemented for ISIC tree, then see if can be moved to superclass
• See if cell overflow tooltips can be created similar to Add tooltip to table cells with content overflow #1130

Relevant PRs:

• Impact categories tree view #485
• Database tree view for databases with ISIC data #1047

[cmutel]

@marc-vdm @mrvisscher Please don't start this independently, we want to add the same functionality for exploring LCA results. Let's do this once, and properly.