Make Timepicker easier to customize

Nobody has claimed this yet.

## Describe the feature

I want to increase the popup size of the TimePicker (maybe even override it in future). For example, this is easily achievable with the Calendar control:

```XAML

<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Calendar}">
<Viewbox Height="450" Width="450">
<Grid x:Name="PART_Root"
HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
Background="Transparent">
<CalendarItem x:Name="PART_CalendarItem"
Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
Style="{TemplateBinding CalendarItemStyle}" />
</Grid>
</Viewbox>
</ControlTemplate>
</Setter.Value>
</Setter>

```

However, it’s much harder with the TimePicker. First of all, you can't do this:
```

```
The problem is that `MahApps.Styles.TimePickerBase` is missing.

Second, I’d like to have more modularity. For example, it would be great if the popup could have a separate style that you could easily override, like:
```
<mah:TimePicker PopupStyle={...}>
```
Otherwise, you need to copy and paste the entire `DateTimePicker.xaml`.

Start by inspecting DateTimePicker.xaml and the existing TimePicker styling, then compare it with the Calendar template shown in the issue. Define how a reusable TimePickerBase and separately overridable popup style would fit the current templates; done means users can resize and customize the popup without copying the entire DateTimePicker.xaml.

Written by the indexing model from the issue text.