Tree widget: Fix main thread blockage on next branch

[JonasDov]

Performance tests on next branch show that main thread is blocked for a long time in some cases. This needs to be fixed after https://github.com/iTwin/viewer-components-react/pull/1511 is merged.

Status | Test                                                                                              | Duration    | Blocks                                                 
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
PASS   | categories tree creates initial filtered view for 50k items                                       | 2680.35 ms  | count: 6, max: 1831.00, p95: 1831.00, median: 31.00    
PASS   | categories tree changing definition container visibility changes visibility for 50k subCategories | 8536.17 ms  | count: 2, max: 6711.00, p95: 6711.00, median: 3483.00  
PASS   | categories tree changing definition container visibility changes visibility for 50k categories    | 12064.57 ms | count: 41, max: 3127.00, p95: 69.00, median: 47.00     
PASS   | classifications tree loads initial view for iModel with 50k classifications                       | 70.92 ms    | count: 0                                               
PASS   | classifications tree loads first branch for iModel with 50k classifications                       | 1935.96 ms  | count: 6, max: 126.00, p95: 126.00, median: 34.50      
PASS   | models tree creates initial filtered view for 50k target items                                    | 628.51 ms   | count: 5, max: 76.00, p95: 76.00, median: 29.00        
PASS   | models tree validates categories visibility for imodel with 50k categories                        | 26182.41 ms | count: 10, max: 11739.00, p95: 11739.00, median: 199.00
PASS   | models tree changing model visibility changes visibility for 50k elements                         | 2032.02 ms  | count: 0                                               
PASS   | models tree changing category visibility changes visibility for 50k elements                      | 2100.9 ms   | count: 0                                               
PASS   | models tree changing per-model-category override changes visibility for 50k elements              | 2419.74 ms  | count: 0