Click targets are too small for some controls

Material Design spec says that all touch targets much be at least 48x48.

mat-checkbox, mat-radio-button, and mat-slide-toggle are only 20x20 (with no label). The slider thumb is also too small.

Adding a padding to the `` element within the template would resolve this (while keeping the visuals unchanged), but would be a major breaking change for people's layouts. We need to explore a non-breaking migration path for this.

This issue is not open for PRs until we have a clear migration strategy. Other components may be too small as well and will need to be audited.