Fixed ListUserItemsRequest performance (#1035)

[reisfmb]

The idea here was a bit more complicated... there are multiple instances of the ListUserItemsRequest class. Here they are:

UserItemsTreeGrid:

• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/UserItemsTreeGrid.ts#L254

PrincipalBrowseFilterPanel:

• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L36
• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L69
• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L99
• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L112

So for instances where the necessary data does not depend on the count parameter, I simply set the count to 1:

• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L36
• https://github.com/enonic/app-users/blob/master/src/main/resources/assets/js/app/browse/filter/PrincipalBrowseFilterPanel.ts#L69

For the remaining instances, I couldn't just set the counter to one, because the data used is actually the users, so by setting counter to N, we'd be showing only N users after some search for example.

Therefore my approach was to set them to 100, and only if the users scroll down the filtered data, then we'll request 100 + N, 100 + 2N, ... number of users.

Why request 100 + N, instead of requesting N and append the response the current data? Because this data comes from a search, so the N new ones might not have anything related to the old ones.

If approved, than I'd like to know how to add the "loading" effect, just like when loading through all users under an idProvider (loadChildren method), that's why this PR is currently a draft.