chore(deps): update dependency materialdesignthemes.mahapps to v3

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
MaterialDesignThemes.MahApps	0.3.0 -> 3.1.0

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Release Notes

MaterialDesignInXAML/MaterialDesignInXamlToolkit (MaterialDesignThemes.MahApps)

v3.1.0: Version 3.1.0

• New filter chips style for ToggleButton by @​mgnslndh #​1456
• Update to .NET Core 3.1 #​1654
• ColorZone now supports a Custom mode by @​ahmedflix25 #​1611
• Pack Icon Changes
• Full list of changes here

ComboBox visual change:

Previously the popup box tried to determine its background color by looking up the visual tree. However this is not always possible (see #​1550). The fix (#​1576) was to switch it to use the theme brush. If you are not using the MaterialDesignPaper theme brush for the control that contains the ComboBox it should just work for you. If you are not doing that, you will notice that the background of your ComobBox drop down is no longer the same color. You can work around this by setting the MaterialDesignPaper resource brush at a reasonable level. Something like:

<ComboBox>
    <ComboBox.Resources>
        <SolidColorBrush x:Key="MaterialDesignPaper" Color="Red" />
    </ComboBox.Resources>
...
</ComboBox>

You can also move that brush up to the Window level resources if it makes sense in your application, rather than specifying it on each ComboBox. Alternatively if you use the same background for all combo boxes you can use this style in your App.xaml

<Style TargetType="ComboBox" BasedOn="{StaticResource {x:Type ComboBox}}">
    <Style.Resources>
        <SolidColorBrush x:Key="MaterialDesignPaper" Color="Red" />
    </Style.Resources>
</Style>

A big thank you to everyone who contributed (either logging issues or submitting PRs):
@​AgPeHaJIuH1
@​ahmedflix25
@​ahmediddma
@​alekstrust
@​AmRo045
@​andybond006
@​awebre
@​bebenins
@​bukshuk
@​burekas7
@​ColossalMoose
@​ContainsK
@​dmuehlenhoff
@​FooPenguin
@​Gopichandar
@​greuelpirat
@​IamJapanese
@​jespersh
@​Jesus805
@​kodaniel
@​kvpt
@​LiptonOlolo
@​mgnslndh
@​MichelMichels
@​mipen
@​mr-at0s
@​Mrxx99
@​NZSmartie
@​paulovila
@​pavka1799
@​philpowers
@​punker76
@​raimand13
@​schorsch2489
@​schulz3000
@​soi013
@​Taschenbuch
@​Wolfteam
@​w-syss
@​xthehacker2000x
@​yangbocheng
@​yosefmah

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