Adv. Inv. restore key bindings

Fixes #9533 - it shall be as it once was:
Page up/down can be used along with </> - page up/down is what *I* would intuitively expect to work, but people are used </> so why not both?

Category movement is done with the up/down keys. See discussion there.
The original was:
```
} else if( action == "PAGE_DOWN" ) {
    if( inCategoryMode ) {
        spane.scroll_category( +1 );
    } else {
        spane.scroll_by( +itemsPerPage );
    }
} else if( action == "PAGE_UP" ) {
    if( inCategoryMode ) {
        spane.scroll_category( -1 );
    } else {
        spane.scroll_by( -itemsPerPage );
    }
} else if( action == "DOWN" ) {
    spane.scroll_by( +1 );
} else if( action == "UP" ) {
    spane.scroll_by( -1 );
```
I moved the `if( inCategoryMode )` block to down into the "DOWN" / "UP" cases:
```
} else if( action == "PAGE_DOWN" ) {
    spane.scroll_by( +itemsPerPage );
} else if( action == "PAGE_UP" ) {
    spane.scroll_by( +itemsPerPage );
} else if( action == "DOWN" ) {
    if( inCategoryMode ) {
        spane.scroll_category( +1 );
    } else {
        spane.scroll_by( +1 );
    }
} else if( action == "UP" ) {
    if( inCategoryMode ) {
        spane.scroll_category( -1 );
    } else {
        spane.scroll_by( -1 );
    }
```
diff sees the three lines `if( inCategoryMode ) {\n...scroll_category...\n} else {` that have apparently not changed at all (they are exactly as before), but the lines around it have changed. This had me confused, too.